Enable `-Wunused-variable` in js/PACKAGE (#49454)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49454

This diff enables compilation warning flags for the directory in question. Further details are in [this workplace post](https://fb.workplace.com/permalink.php?story_fbid=pfbid02XaWNiCVk69r1ghfvDVpujB8Hr9Y61uDvNakxiZFa2jwiPHscVdEQwCBHrmWZSyMRl&id=100051201402394).

This is a low-risk diff. There are **no run-time effects** and the diff has already been observed to compile locally. **If the code compiles, it work; test errors are spurious.**

Differential Revision: D68513114

fbshipit-source-id: 219e2b9f2961da76cec4d08cc9f77787e08a0a1e